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Topic: CARBURETTOR Problems In Low RPMs (Read 7055 times)

I'm going bananas over a carb problem in my 2002 FZS600. The engine idled perfectly but had power retardation in the top gear when overtaking. The carbs have been taken apart twice now, the first time to be cleaned out because there was rust in the tank. All the jets and passages cleaned and blown out. The bike didn't run the same afterwards so they were taken apart again, o-rings and needle valves replaced, floats tested for leaks, fuel level set, carbs balanced by a shop but still the bike doesn't run properly!

It sounds like it doesn't run on all 4 when idling and in low RPMs, but mid range and top is great. It passed MOT today and since I was at the shop they checked the temperature on each cylinder, 3 & 4 are running much cooler than the rest.

wait for it dark then spray some water over the ht leads and cap and see if you can see very small sparks if you do then you need to change them hopefully it will be the caps if not new or second hand set off coils

As above, new spark plugs and/or caps will likely sort that. Cylinders 3 & 4 are usually exposed to the elements if your bike spends anytime outside on the side stand. Spark plugs corrode, and a build up of dirt, water and corrosion around them doesn't help.

As suggested I would check HT leads,especially making sure that connectors are pressed fully home onto plug tops. i mention this because I ,like yourself ,recently had my carbs off for overhaul & to fit new intake rubbers.When all back together bike would "bog down"if given a handful of throttle but would eventually "catch up" as the revs built up but wouldnt pull like it should.I ASSUMED carb fault but trying to use a bit of logic decided to remove plug caps,spray inside with a bit of ACF50 and discovered n02 cap wasnt pressed fully home. Result-everything now running right! [size=78%] Its expecting a lot of a 4 pot motor to run 1-MISS-4-3.[/size][/size][size=78%] [/size]

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Grandaughter; "When are you going to grow up Grandad?"Me:" Growing old-cant avoid that-Growing up that's OPTIONAL!"

There's loads of things that could cause this effect unfortunately. It's just a process of elimination one at a time.Bit confusing the way you described that the bike is running oppositely now to when you first had issues before carb cleaning though.

Firstly you've not mentioned the air filter at all, when was it last changed and what type/brand do you have?If the bike can't breathe properly then it will affect the idle and lower revs, but can also hold you back at higher revs if it's particularly dirty.Have you even looked into the airbox, it could be clogged up with emulsified oil as well, which my bike has had before.

A carb balance by the way will only work well when everything else is running properly.So if your air filter is clogged (or you have an air leak), your TPS is breaking down or knackered, or your spark plug caps are on their way out then a carb balance will be irrelevant and can even make things worse.Even the TPS positioned wrongly can make the bike feel a tad jerky at lower revs, i would do a check on that too, i'm on my 4th one now although this time i bought a brand new one at a much lower price than over here from Germany as it was all getting a bit silly buying used ones that were at least 13 years old and likely to peg out at any time.http://www.ebay.co.uk/itm/Drosselklappensensor-TPS-101-Yamaha-FZS-600-DROSSELKLAPPE-13550-13D60-4HD-85885-/361403667980?hash=item542556560c:g:svIAAOSw2s1UxgsX

Another small possibility is moisture in the tank, which will sit lower down in the carbs and cause roughness at idle and low revs but once sloshing away at higher speed won't be as noticeable.

wait for it dark then spray some water over the ht leads and cap and see if you can see very small sparks if you do then you need to change them hopefully it will be the caps if not new or second hand set off coils

As above, new spark plugs and/or caps will likely sort that. Cylinders 3 & 4 are usually exposed to the elements if your bike spends anytime outside on the side stand. Spark plugs corrode, and a build up of dirt, water and corrosion around them doesn't help.

The bike has been kept inside for the last year, the year before it was outside but on the centre stand and usually with a rain cover. But it probably had it's share of exposure to the elements since the #3 spark plug was stuck in the cylinder head. Several months of soaking it in WD40 and working it back and forth did the trick. When the carbs were put back in for the second time new plugs were fitted. I'm definitely going to check the caps and the leads!

Firstly you've not mentioned the air filter at all, when was it last changed and what type/brand do you have?If the bike can't breathe properly then it will affect the idle and lower revs, but can also hold you back at higher revs if it's particularly dirty.Have you even looked into the airbox, it could be clogged up with emulsified oil as well, which my bike has had before.

A carb balance by the way will only work well when everything else is running properly.So if your air filter is clogged (or you have an air leak), your TPS is breaking down or knackered, or your spark plug caps are on their way out then a carb balance will be irrelevant and can even make things worse.Even the TPS positioned wrongly can make the bike feel a tad jerky at lower revs, i would do a check on that too, i'm on my 4th one now although this time i bought a brand new one at a much lower price than over here from Germany as it was all getting a bit silly buying used ones that were at least 13 years old and likely to peg out at any time.http://www.ebay.co.uk/itm/Drosselklappensensor-TPS-101-Yamaha-FZS-600-DROSSELKLAPPE-13550-13D60-4HD-85885-/361403667980?hash=item542556560c:g:svIAAOSw2s1UxgsX

I did check the air filter, it was a bit dirty, not too much though and I managed to shake and brush away most of the dirt out, I put it back in because I didn't have a new one at the time. The box is clean but the small breather filter had some gunk in it which I cleaned out. Rubber inlets are intact. I'm gonna get a new air filter just in case, should I get the OEM one like the one I have or should I go the K&N route? Or is the high flow of the K&N going to screw everything up more?

Thanks so much for the link to the plug caps, they are at a great price! I did notice though that they don't have the 45 degree (more or less) elbow but I guess you didn't have issues with that since it has been done many times.

I checked the position of the TPS with the speedometer, it showed a tad over 5000 RPM so I twisted it back on forth and made it land on 5000 but theres a lot of leeway on both sides before the RPM needle drops up or down. How can you tell if the sensor is failing? You mentioned that you're on the 4th now, did you measure the resistance in the faulty one when replacing it with a new one?

Last but maybe fundamentally not least, the last fuel level measurement showed that carb #3 & #4 have about 1mm higher fuel level, could this tiny difference cause all this uneven running? Like I mentioned initially, cylinder #3 & #4 are running cooler that the rest, what's the logic here, is it because they don't get enough fuel or is it because they are getting too much fuel? Gonna check the spark plugs for colour later to get some indication.

wait for it dark then spray some water over the ht leads and cap and see if you can see very small sparks if you do then you need to change them hopefully it will be the caps if not new or second hand set off coils

As above, new spark plugs and/or caps will likely sort that. Cylinders 3 & 4 are usually exposed to the elements if your bike spends anytime outside on the side stand. Spark plugs corrode, and a build up of dirt, water and corrosion around them doesn't help.

The bike has been kept inside for the last year, the year before it was outside but on the centre stand and usually with a rain cover. But it probably had it's share of exposure to the elements since the #3 spark plug was stuck in the cylinder head. Several months of soaking it in WD40 and working it back and forth did the trick. When the carbs were put back in for the second time new plugs were fitted. I'm definitely going to check the caps and the leads!

Firstly you've not mentioned the air filter at all, when was it last changed and what type/brand do you have?If the bike can't breathe properly then it will affect the idle and lower revs, but can also hold you back at higher revs if it's particularly dirty.Have you even looked into the airbox, it could be clogged up with emulsified oil as well, which my bike has had before.

A carb balance by the way will only work well when everything else is running properly.So if your air filter is clogged (or you have an air leak), your TPS is breaking down or knackered, or your spark plug caps are on their way out then a carb balance will be irrelevant and can even make things worse.Even the TPS positioned wrongly can make the bike feel a tad jerky at lower revs, i would do a check on that too, i'm on my 4th one now although this time i bought a brand new one at a much lower price than over here from Germany as it was all getting a bit silly buying used ones that were at least 13 years old and likely to peg out at any time.http://www.ebay.co.uk/itm/Drosselklappensensor-TPS-101-Yamaha-FZS-600-DROSSELKLAPPE-13550-13D60-4HD-85885-/361403667980?hash=item542556560c:g:svIAAOSw2s1UxgsX

I did check the air filter, it was a bit dirty, not too much though and I managed to shake and brush away most of the dirt out, I put it back in because I didn't have a new one at the time. The box is clean but the small breather filter had some gunk in it which I cleaned out. Rubber inlets are intact. I'm gonna get a new air filter just in case, should I get the OEM one like the one I have or should I go the K&N route? Or is the high flow of the K&N going to screw everything up more?

Thanks so much for the link to the plug caps, they are at a great price! I did notice though that they don't have the 45 degree (more or less) elbow but I guess you didn't have issues with that since it has been done many times.

I checked the position of the TPS with the speedometer, it showed a tad over 5000 RPM so I twisted it back on forth and made it land on 5000 but theres a lot of leeway on both sides before the RPM needle drops up or down. How can you tell if the sensor is failing? You mentioned that you're on the 4th now, did you measure the resistance in the faulty one when replacing it with a new one?

Last but maybe fundamentally not least, the last fuel level measurement showed that carb #3 & #4 have about 1mm higher fuel level, could this tiny difference cause all this uneven running? Like I mentioned initially, cylinder #3 & #4 are running cooler that the rest, what's the logic here, is it because they don't get enough fuel or is it because they are getting too much fuel? Gonna check the spark plugs for colour later to get some indication.

Thanks for all the replies!

I'm a big fan of the K&N air filter, it makes the throttle feel much more responsive in my opinion, and the bike generally feels like it can breathe properly. They do need cleaning more than it says on the box though, but I learned after a bit of research that washing powder has similar ingredients of the K&N own brand stuff and I've tried it and it works fine, but I still use the K&N oil to spray on once it's dry.

The straight NGK caps have no issues at all, just a matter of preference I s'pose.

Around the 5000rpm mark on the speedo is fine for the TPS.Just from experience I now even know when the TPS is not set in the right position, the bike does what I can only describe as a shivering effect, it's mild but when you know your own bike it's noticeable. It will tend to keep going out of position over time when it is failing, then with mine they eventually wouldn't land on the 5000rpm mark, the last one was stuck on 10,000rpm. Plus the bike by now will generally run rough at lower revs, normally anywhere up to 3000rpm, nothing major but again just not right. It feels like it's okay above that but it isn't really, it's just the bike noise and speed covering it up I reckon.

As for different fuel levels in the carbs, is that not all part of balancing them out so they get equal fuel?Like I said earlier if other things aren't behaving or correct then balancing the carbs will give a false reading making the carbs unbalanced.I did it myself with a stuffed up air filter and the bike ran like crap.Cleaned the air filter and balanced them again a week later and the CarbTune was well out, but once levelled again it all worked lovely.

It sounds like it doesn't run on all 4 when idling and in low RPMs, but mid range and top is great. It passed MOT today and since I was at the shop they checked the temperature on each cylinder, 3 & 4 are running much cooler than the rest.

That's just how mine was running up until a few weeks ago (except it was only cooler on down-pipe number 3).

The workshop did an ultrasonic clean of the carbs and also fitted a new set of spark plugs.

Apparently the main culprit was a blocked primary jet in number 3 carb.

It sounds like it doesn't run on all 4 when idling and in low RPMs, but mid range and top is great. It passed MOT today and since I was at the shop they checked the temperature on each cylinder, 3 & 4 are running much cooler than the rest.

That's just how mine was running up until a few weeks ago (except it was only cooler on down-pipe number 3).

The workshop did an ultrasonic clean of the carbs and also fitted a new set of spark plugs.

Apparently the main culprit was a blocked primary jet in number 3 carb.

I have changed the plug caps which didn't make a difference but at least now I'll know I won't get a misfire from there. While I was at it I took the plugs out and #4 was a tiny bit whiter than the rest. If I recall correctly then that's a sign of a lean mixture.

I think I will finally find some time tomorrow to rip the carbs off once again and inspect those jets. They have been cleaned with a carb cleaner twice already and air did flow through them and the channels in the carbs.

So could there be an accumulation of dirt that formed some kind of a lime scale that could make those narrow channels even smaller to a point of being too narrow and the only way to clean them out would be by ultrasonic? Was it the cleaning of the jets or the cleaning of the carbs that solved the problem?

I didn't read all the thread so this might have been suggested already but you said the initial problem was a rusty tank? Did you treat the rust in the tank? Or did you solve why it was going rusty in the first place? Was there water getting into it? You might have more water in there or more rust has got through clogging up the pilot jets. I think there's a filter in the tank too did you clean that? And did you replace the inline filter? If you've had rust problems in the past I'd be leaning towards a fuel supply issue than a spark issue

I didn't read all the thread so this might have been suggested already but you said the initial problem was a rusty tank? Did you treat the rust in the tank? Or did you solve why it was going rusty in the first place? Was there water getting into it? You might have more water in there or more rust has got through clogging up the pilot jets. I think there's a filter in the tank too did you clean that? And did you replace the inline filter? If you've had rust problems in the past I'd be leaning towards a fuel supply issue than a spark issue

Yes the rust was treated and a new inline fuel filter fitted, the fuel looks very clean so far. There's a filter in the tank, where the hell is that? This is definitely a fuel problem and I'm leaning towards the pilot jets as it's only in the lowest of RPMs. Has anyone had to replace them?

There's one on the fuel tap in the tank. Come to think of it I had a mate who bought a bike with a rusty tank and he treated the tank with a sealer. It was running fine off a fuel bottle but it wouldn't idle when the tank was hooked up. Maybe he sealed up the filter when he was sealing the tank

There's one on the fuel tap in the tank. Come to think of it I had a mate who bought a bike with a rusty tank and he treated the tank with a sealer. It was running fine off a fuel bottle but it wouldn't idle when the tank was hooked up. Maybe he sealed up the filter when he was sealing the tank

Just looking on the AJ Sutton site, I'm presuming you can't buy that filter as it's not listed as a separate item, just joined to the tap assembly?

You can pop them out and clean them. It's just an o ring holding them in. If you googled fuel tap filter you might find one. It might not be your problem, I think you'll have to check the pilot jets again but it's another thing to rule out

[/quote]So could there be an accumulation of dirt that formed some kind of a lime scale that could make those narrow channels even smaller to a point of being too narrow and the only way to clean them out would be by ultrasonic? Was it the cleaning of the jets or the cleaning of the carbs that solved the problem?[/quote]

The poor running issue with my bike was caused by degraded and/or evaporated petrol which had left deposits in the carbs.

The workshop said that the main cause was the blocked pilot jet in number 3, but a general accumulation of muck was also contributing to it. The ultrasonic cleaning sorted it all out.

It was a consequence of not riding/refuelling often enough.

I also seem to remember reading somewhere that if you always store your bike on the side-stand for long periods it's more likely that this issue will occur in carbs 3&4.

So I managed to spend some time on the carbs today, too much for my liking as I'm going nuts over this. None of the jets looked blocked but I sprayed everything with carb cleaner, let it soak for a bit and then blew through all the passages I could find.

This didn't change a thing, the engine idles like a sick dog. Could the passage for the pilot jet be clogged, what's the best way to clean it? Since the carb cleaner didn't work is the ultrasonic the only way?

Where can I get ultrasonic cleaning, do most work shops provide this service? Doesn't the whole carb need to be submerged in order to ultrasonically clean the passages?

How much does utrasonic cleaning cost?With the pain you're going through i'd be inclined to consider either handing it over to a pro with the bike and let them sort it, or even buying a used set of carbs.I know that's not ideal, especially if you prefer to do stuff yourself and don't want to be beaten by it, but i'm personally too impatient and would want it sorted asap.

I dunno where you are but i've always given my bike to these lot when i've had carb/running troubles, not cheap but they certainly get the job done properly while you wait.

How much does utrasonic cleaning cost?With the pain you're going through i'd be inclined to consider either handing it over to a pro with the bike and let them sort it, or even buying a used set of carbs.I know that's not ideal, especially if you prefer to do stuff yourself and don't want to be beaten by it, but i'm personally too impatient and would want it sorted asap.

I dunno where you are but i've always given my bike to these lot when i've had carb/running troubles, not cheap but they certainly get the job done properly while you wait.

Yeah I like to do stuff myself since I can do a good job but I haven't done many carbs. I understand how they work but I prefer much more electronic injection as it's more reliable and you can do a custom mapping instead of painful jetting.

I'm rarely beaten by mechanical things but last month I gave up on this and took the bike to a shop for carb balancing as I thought that might be a solution but the bike came out worse and was drowning in fuel, you could just smell it. I wasn't very happy and obviously took it back but now it's just back to how it was, waste of money that was.

Used set of carbs is something I have thought about already but they all look so filthy on eBay so it would be just another cleaning job and probably a risk of getting a carb that is in a worse state than mine.

I have almost totally lost my patience on this so I called PDQ (thanks for the suggestion) and they are booked until the third week of September! I had a long chat with one of their guys though and he confirmed my suspicion about the passage that lies beneath the pilot jet. He didn't recommend ultrasonic cleaning though, old school elbow grease is much better he said. He recommended getting a nylon wire or something small into the passage, give it a scrub with carb cleaner and blow it out. Hopefully that is going to save my mentality!

How much does utrasonic cleaning cost?With the pain you're going through i'd be inclined to consider either handing it over to a pro with the bike and let them sort it, or even buying a used set of carbs.I know that's not ideal, especially if you prefer to do stuff yourself and don't want to be beaten by it, but i'm personally too impatient and would want it sorted asap.

I dunno where you are but i've always given my bike to these lot when i've had carb/running troubles, not cheap but they certainly get the job done properly while you wait.

Yeah I like to do stuff myself since I can do a good job but I haven't done many carbs. I understand how they work but I prefer much more electronic injection as it's more reliable and you can do a custom mapping instead of painful jetting.

I'm rarely beaten by mechanical things but last month I gave up on this and took the bike to a shop for carb balancing as I thought that might be a solution but the bike came out worse and was drowning in fuel, you could just smell it. I wasn't very happy and obviously took it back but now it's just back to how it was, waste of money that was.

Used set of carbs is something I have thought about already but they all look so filthy on eBay so it would be just another cleaning job and probably a risk of getting a carb that is in a worse state than mine.

I have almost totally lost my patience on this so I called PDQ (thanks for the suggestion) and they are booked until the third week of September! I had a long chat with one of their guys though and he confirmed my suspicion about the passage that lies beneath the pilot jet. He didn't recommend ultrasonic cleaning though, old school elbow grease is much better he said. He recommended getting a nylon wire or something small into the passage, give it a scrub with carb cleaner and blow it out. Hopefully that is going to save my mentality!

Yeah i forgot to mention PDQ are very human over the phone as well as in the workshop, they actually do give a shit about bike problems and will point you in the right direction.Gotta remember the age of these bikes so unless you buy new then all carbs are gonna look grubby, it's what's going on inside that counts though.

For the record, a new set of carbs is £611, but you do get a free delivery

Gotta remember the age of these bikes so unless you buy new then all carbs are gonna look grubby, it's what's going on inside that counts though.

For the record, a new set of carbs is £611, but you do get a free delivery

Yeah that's exactly what I'm afraid of, the inside could be worse than the outside! Green accumulated petrol gunk

That's very kind of them, FREE DELIVERY! It would be the most absurd thing I could do as the bike costed less than that! It was a salvage bike which I bought as a project bike, was about to finish a big mod when the running problem accrued and it has delayed the works for the whole summer as I don't have the heart nor time to continue until I sort out the heart of the beast!

Since I just finished 30K and didn't get the bike's history when I bought it I decided to rule out the valve clearance. When I got the bike it had 22K on the clock and I had no idea if the clearance was checked or not. So on Sunday I decided to open up the engine and check, all measurements well within specs and timing markings spot on so that has been ruled out.

During the weekend I took the jets out and soaked them in carb cleaner over night, I also took the carbs to a local shop and used an air nozzle connected to their compressor and blow out all the passages because so far I've been used canned air but obviously provides less air pressure. Nice air flow through all the passages.

After a night's worth of soaking in carb cleaner I still noticed dark spots on the outside of some jets so I decided to soak them another night but this time in Pepsi. Next morning the jets were amazingly shiny! My intestines must be super clean as well, haha!

Very happy with my new inexpensive carb and stomach cleaner I put the carbs back together, added some fuel and realised that the fuel level was all over the place!!!I haven't touched the tangs on the floats since the last time so WTF was going on?!? I opened up the carbs again, bent the tangs all the way up (towards the needle valve) and still I would have way too much fuel on carb 1 & 3

Dont know if this helps but I recently ordered 4 carburettor overhaul kits from M & P direct. They came in packaging showing maker "Tourmax"made in Japan suitable for FZS 600 1998-2001. ref no 7245384 Vergaser (German for carb) Rep STZ CAB-Y73. Cost for four inc p&p £42-95.Fitted fine & bike running well. Cant say I noticed any markings on actual needle valve assemblies but my eyes are as old as the rest of me-dont ask! I can understand variation in size of main,pilot,starter jets but why would there be a need for different sizes of the needle valve?Sounds like you have an over supply of fuel on 2 carbs-maybe recheck condition of "O" rings,floats, & float heights on offending carbs.

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Grandaughter; "When are you going to grow up Grandad?"Me:" Growing old-cant avoid that-Growing up that's OPTIONAL!"

2 further thoughts:1. You did tighten the needle valve seat retaining screws when you changed them?2.Float height for 2000 on models should be 3.5mm above float chamber mating surface ( Its 4.5mm on earlier models) 2000 on carbs are marked 5DM1 01Earlier models are 5DM1 00.Only mention this in case carbs have been replaced by previous owner. I know its a bit of a long shot but in diagnosing faults you cant assume anything. Dont mean to teach anyone to suck eggs but it seems you have ruled out a lot of the obvious things mentioned by others. Good luck!

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Grandaughter; "When are you going to grow up Grandad?"Me:" Growing old-cant avoid that-Growing up that's OPTIONAL!"

Dont know if this helps but I recently ordered 4 carburettor overhaul kits from M & P direct. They came in packaging showing maker "Tourmax"made in Japan suitable for FZS 600 1998-2001. ref no 7245384 Vergaser (German for carb) Rep STZ CAB-Y73. Cost for four inc p&p £42-95.Fitted fine & bike running well. Cant say I noticed any markings on actual needle valve assemblies but my eyes are as old as the rest of me-dont ask! I can understand variation in size of main,pilot,starter jets but why would there be a need for different sizes of the needle valve?Sounds like you have an over supply of fuel on 2 carbs-maybe recheck condition of "O" rings,floats, & float heights on offending carbs.

The only reason I could think of is to get a faster fuel flow when racing, if the throttle is pretty much wide open most of the time then the bowls would fill up faster with the bigger needle valves. The "O" rings are just a few months old and fine, the floats have been left submerged in fuel and came up without a drop inside so they don't seem to be leaking, the tangs on the floats are bent way much to my liking and compared to how they were originally.

I remember that it was a massive pain to get the fuel level right, both with the original and the new needle valves. One minute I almost had it and after a minor adjustment it changed drastically like something was leaking. Could there be a crack in the carb body?